The concept of the home deployed base station, or femtocell, is considered of increasing importance for cellular network operators. Femtocells operate at low downlink transmit power, and are designed to improve the cellular coverage within a home or enterprise environment and their immediate surroundings. They would typically be linked into the wider cellular Radio Access Network through a customer's broadband link (e.g. digital subscriber line, cable, passive optical network or other wireline access technology).
Although generally placed indoors, femtocells operate within an existing conventional cellular wireless network, which is termed a macrocellular network. If the femtocells are configured for open access, (that is, where all users who are capable of receiving a signal from a given femtocell can use it to provide cellular access irrespective of whether or not they are the owner of the femtocell in question), then the number of potential base stations through which a user can connect to the network is greatly expanded to cover all those macrocells and femtocells with sufficiently high received signal-to-interference-and-noise ratio. Femtocells may in principle be operated in a separate frequency band from that used by macrocells to minimise interference between the two systems, but this involves the use of additional licensed spectrum; in general femtocells are therefore operated in the same frequency band as is used by macrocells.
The term “base station” is used here to refer to a radio transceiver connected to a telecommunications network; a cell site may have several base stations, each serving a different area of wireless coverage. This deployment of multiple base stations at a cell site is particularly common for macrocellular networks, but typically femtocell base stations are proposed to be used individually, with an omni-directional antenna. The user equipment terminal, often a mobile device, is referred to simply as a “user equipment”.
The use of femtocells is particularly applicable to high capacity packet data cellular wireless communication systems such as HSPA (‘High Speed Packet Access’), a so-called third generation evolutionary system, and LTE (Long Term Evolution), often referred to as a fourth generation (4G) systems. Applications used with such systems can typically accommodate a variable data rate to and from the user equipment, and can exploit a greater data rate should it be available, for example for the faster transfer of data files. It is accordingly advantageous to maximise the data capacity available to a user, and to this end adaptive modulation and coding is typically employed. The provision of a femtocell within a subscriber's premises can provide a high capacity link within a small local area that will typically also be within the coverage area of a macrocell.
The use of Best Server Selection (BSS) has been proposed for the HSPA system and is also applicable to the LTE systems. This refers to a method by which the base station, also referred to as a server and used for the transfer of data to and from a user equipment, is selected from a so-called active set on a packet by packet basis. That is to say, signalling is maintained between each user equipment and an active set of base stations (provided that signals can be received from more than one base station), but payload data is only sent between the user equipment and one of the active set to achieve efficient use of the radio resource. The base station with which the highest quality of signals is exchanged is typically selected on the basis of the capacity of the respective links, this being dependent on the adaptive modulation and coding provided thereby. This is in contrast to third generation systems such as UMTS release 99, in which so-called soft handover is used, and involves several base stations within an active set simultaneously transmitting the same payload data to a user equipment. The user equipment then combines the payload data using a combining algorithm; this is a robust system but has less potential data capacity than the BSS approach.
As is well known in the art, the concept of handover refers to the process of changing the base station or set of base stations that communicate payload data with a user equipment. The process of handover has evolved between generations of cellular wireless; first and second generation systems employed what may be termed hard handover, in which data payload communication was transferred from a single base station to another base station. In third generation systems, as already discussed, an active set of base stations is maintained where possible, and handover relates to a change in the selection of the active set. In third generation evolutionary systems and fourth generation systems, handover again relates to the selection of an active set of base stations but in addition there is a process of selection of the best base station within the active set for data payload transfer, potentially on a packet-by-packet basis, a process known as re-pointing.
In the handover process, it is conventional to select base stations on the basis of received signal quality. In the case where base stations serve a large number of user equipment terminals, it is typically assumed that the loading of each base station in terms of the bandwidth demands of the user equipment is reasonably equal, so the selection of base stations on the basis of signal quality is a reasonable approach to allocating resource.
However, in the case of potential handover and re-pointing between femtocells and macrocells the situation is different. In this case there may be a large imbalance between the number of user equipments served by a macrocell base station and a femtocell base station: for example, there may be a hundred user equipments served by a macrocell base station but only one or indeed no user equipments served by a femtocell base station at a given time. There may be a situation in which a user equipment receives a strong signal from a macrocell base station and a weaker signal from a femtocell base station. It would be expected that the operation of adaptive modulation and coding will typically enable a higher channel data rate using the strongest signal, but this data rate is shared between the user terminals communicating with the base station. It can be seen that a handover or re-pointing selection on the basis of signal quality may not maximise the data capacity available to the user equipment in this situation.
Various workers have documented the criteria used to perform soft and/or hard handover in various network environments. For example, the concept of using base station loading conditions, as opposed or in addition to signal quality information, as a basis for hard handover decisions between cells in a macrocellular network is known from International Patent application having publication number WO2004/019643. A method of managing wireless LAN resources for performing dynamic allocation of bandwidth to passing mobile terminals in dependence on the fixed subscribers instantaneous traffic load over the wireless LAN and over the broadband access line is described in international patent application having publication number WO2004/034646. The idea of allocating mobile stations to an unlicensed mobile network controller (UNC) in an unlicensed mobile radio access network is described in international patent application having publication number WO2006/024887. In the arrangement described in this application, a UNC is logically divided into differing roles (provisioning, default and serving) and a mobile station is assigned to a UNC on the basis of operation, maintenance and configuration activities in the network so as to improve load balancing in the network.